A magnetic recording medium basically consists of a non-magnetic base made of polyethylene terephthalate film, polycarbonate film or polyimide film, and an overlying magnetic layer principally made of ferromagnetic particles and a binder. Fine particles of oxide-based compounds have been conventionally used as the ferromagnetic particles. Today, ferromagnetic metal particles having high saturation magnetization and coercive force are used for the purpose of providing increased magnetic recording density and reproduction output. Although metallic magnetic materials have desired magnetic characteristics, they have low chemical stability and are easily oxidized and attacked by corrosion. As a result, a magnetic recording medium using these materials does not perform consistently over an extended period of time. The metallic magnetic particles also have high saturation magnetization (.sigma.s). Furthermore, due to strong interaction between the individual particles, a dispersion of the particles cannot be easily formed and once formed the dispersion does not remain stable for a sufficient period of time. As a further disadvantage, the metallic magnetic particles are highly inflammable in air and present handling problems during transport and manufacturing. It is well known that the smaller the grains of a magnetic recording medium, the higher the recording density. But then, the problems described above become conspicuous.
To eliminate these defects, several methods have been proposed and some of them are currently used on a commercial basis. They include:
(a) Formation of a modified layer such as oxide, sulfide or nitride on the surface of magnetic particles (Japanese Patent Publication No. 3862/60, Japanese Patent Application (OPI) Nos. 54948/77, 85054/77, 149458/76, 112465/76 (the symbol OPI as used herein means an unexamined published Japanese application), U.S. Pat. Nos. 4,113,528, 3,700,499, 3,904,448, 3,960,569, Japanese Patent Application (OPI) Nos. 123601/74 and 5038/78);
(b) Formation of an aliphatic acid, aliphatic acid salt, complex, coupling agent, surfactant or polymer on the surface of magnetic particles by means of adsorption or chemical bonding (Japanese Patent Publication No. 20116/68, Japanese Patent Application (OPI) Nos. 97738/74, 135835/74, 8798/78, 109498/76, 72498/77, 119696/79, 77270/79, 12958/72 and U.S. Pat. 4,063,000);
(c) Polymerization on the surface of magnetic particles or direct deposition of a polymer on their surface (U.S. Pat. No. 3,480,553, Japanese Patent Application (OPI) Nos. 102606/76, 78099/78 and 39662/80); and
(d) Formation of a metal coating on the surface of magnetic particles by electroless plating or replacement plating (Japanese Patent Application (OPI) Nos. 60800/75, 67250/76, U.S. Pat. Nos. 3,892,600, 3,856,581, 3,856,580, 3,853,640, 3,892,599, 3,856,582 and 3,892,601.
However, none of the conventional methods are sufficiently effective to solve the problems with the metallic magnetic particles. In method (a), if the thickness of the modified layer is increased to achieve greater effect, the magnetic characteristics of the particles are reduced. Therefore, it is difficult to have a good balance between the thickness of the modified layer and the magnetic characteristics of the particles. In method (b), strong adsorption is difficult to obtain, and if a highly reactive substance is selected, it may react with the constitutent metal atom of the magnetic powder when it is blended with a binder and as a result, the reaction product may dissolve out in the dispersion. In method (c), a thick polymer coating has a tendency to reduce the magnetic characteristics of the particles or to agglomerate the particles when they are subjected to a subsequent treatment. The metal coating formed by the method (d) often has a corrosive effect on the surface of the metallic magnetic particles thereby greatly reducing their characteristics.